[ES|QL] Support multivalues (or comma separated values) in named params

[stratoula]

Description

Right now named params in ES|QL support only one value. So if you want to support somehow multivalues in named params you can't.

You can have atm something like

WHERE field IN (?param1, ?param2)

but you can't have something like

WHERE field IN (?param)

where ?param takes comma separated values.

In a more real example, we would like at kibana to use named params in a query like this

FROM logst* | WHERE extension.keyword IN ("css", "jpg")

We want the user here to be able to send the "css", "jpg" in one named param. This will enable multivalues support at kibana ES|QL controls

Other use cases

Other cases would be a multifields support in the STATS ... BY where you can have something like

.... BY ?breakdown

and the user gives multiple fields, comma separated

The same for KEEP and DROP

Also I want to do

FROM ?sources

and the sources to be a comma separated list of index patterns

[elasticsearchmachine]

Pinging @elastic/kibana-esql (ES|QL-ui)

[elasticsearchmachine]

Pinging @elastic/es-analytical-engine (Team:Analytics)